The present invention relates to image pickup apparatus in which image signals corresponding to a plurality of frames at different exposure amounts are outputted from an image pickup device and are synthesized to obtain an image having wide dynamic range, and also relates to apparatus related thereto.
Solid-state image pickup devices such as CCD image pickup device are generally used in image pickup apparatus, as TV camera, video camera, electronic camera, etc. There is a problem however that the dynamic range of a solid-state image pickup device is much narrower than that of a silver salt photographic film.
To eliminate this problem, there have been proposed techniques in which image signals corresponding to two frames at different exposure amounts are read out from a single image pickup device and are synthesized to obtain an image having an increased dynamic range. For example, an image pickup apparatus having the following construction is disclosed in Japanese Patent Publication No. 2522015. In particular, a disclosure has been made with respect to an image pickup apparatus including: image pickup means for converting object image into electrical signals; image pickup control means for selectively executing a first mode or second mode, said first mode for successively outputting images at different exposure amounts in cycles by cyclically changing accumulating time at the image pickup means, image portions at suitable level being synthesized to form a synthesized frame of said predetermined cycle by respectively comparing signal level of each portion of a plurality of frames at different exposure amounts outputted from said image pickup means with a predetermined reference value, said second mode for providing output by using a constant accumulating time at said image pickup means; and switch control means for switching the image pickup control means to the first mode when an existence of object having a large difference in luminance within a frame is detected in said second mode by comparing signals obtained from said image pickup means with a predetermined signal level. This image pickup apparatus, when in the first mode, is capable of obtaining an image of which all the portions of the frame are at suitable signal level. Even when an object having a large difference in luminance exists within a frame as in a backlighted condition, lack of detail at high level portion for example does not occur within the frame and it is in effect possible to widen dynamic range thereof.
Further, without a determination made by the operator on the conditions of the object, etc., switching to the first mode is automatically made by the switching control means when an object having a large difference in luminance exists within the frame in the second mode, making it possible to automatically correct a backlighted condition, etc.
Further, a disclosure has been made in Japanese patent application laid open No. 5-64075 with respect to a video camera including: image pickup means including a solid-state image pickup device, for outputting image signals for representing an object image; motion detection means for determining based on the image signals obtained from the image pickup means whether a motion exceeding a predetermined amount occurs in the object or not; means for controlling exposure amount so that the image pickup means takes images of the object at two different exposure amounts on condition that a motion has not been detected by the motion detection means; and means for forming synthesized image signals by replacing, of the image signals representing an image at the larger exposure amount, those image signals representing regions of relatively high luminance in that image with those image signals representing the corresponding regions in an image at the smaller exposure amount. Also, the same publication discloses a video camera which furthermore includes high luminance region detection means for determining whether or not, of the above described image signals for representing an image at the larger exposure amount, the regions of relatively high luminance in that image exceed a predetermined area. The above described exposure amount control means controls the exposure amount so that images can be picked up at two different exposure amounts on condition that no motion has been detected by the above motion detection means and that a high luminance region has been detected by the above described high luminance region detection means.
In the above described image pickup apparatus as disclosed in Japanese Patent Publication No. 2522015, however, the mode switching means for switching to the first mode for generating image having a wide dynamic range is to switch the mode when an existence of object having a large difference in luminance is detected within a frame obtained in the second mode. The disclosure merely refers to the switching of mode depending on the conditions only of luminance values and other conditions concerning the generation of wide dynamic range image were not at all taken into consideration.
Further, in Japanese patent application laid open No. 5-64075, motion of the object is detected from image signals after image taking and a synthesized image is obtained by picking up images at two different exposure amounts on condition that motion of the object is absent. Thus the detecting of motion of the object from something other than the image signals before image taking was not taken into consideration. Other conditions for generating a synthesized image except the detection of motion of the object were not taken into consideration either.